Yosemite has been on my bucket list for quite a while, but sadly I have not yet been able to visit it in person. Tonight while reading the news I came across a report of a child contracting plague (Yersinia pestis) in the Yosemite National Park (1) (2). Details are quite sparse but new reports indicate that this is the first human case of Plague in California since 2006. There have also been some recent cases in the state of Colorado.

In 2012 Yosemite was in the news for another infectious disease, Hantavirus associated visitors who stayed in the Curry Village Tents and High Sierra Camps in July of 2012 (3). Hantavirus is a single-stranded, enveloped, negative sense RNA viruses in the Bunyaviridae family (more on that in an upcoming blog post)(4)(5).

Like plague (Yersinia pestis) Hantavirus is also spread by contact with infected rodents, primarily deer mice.

Plague (Yersinia pestis) has a few main means of transmission which include direct physical contact, indirect contact with contaminated soil or surfaces, fecal-oral transmission from contaminated food or water and vector borne transmission (carried by fleas from rodents to other mammals). The most rare and feared potential modes of transmission are droplet contact or airborne transmission though the evidence for this means of transmission in recent outbreaks is quite low.

Perry and Fetherston (1997) said:

Plague is a zoonotic disease primarily affecting rodents; humans
play no role in the long-term survival of Y. pestis. Transmission
between rodents is accomplished by their associated
fleas. While infection can occur by direct contact or ingestion,
these routes do not normally play a role in the maintenance of
Y. pestis in animal reservoirs. Fleas acquire Y. pestis from an
infected blood meal. Infection in the flea is restricted to the
alimentary canal with other organs and tissues including salivary
glands, reproductive organs, and the hemocele being unaffected.

While doing some reading on this topic I also learned a new term called transovarial transmission, which means transmission of a pathogen from an organism to their offspring by infection of eggs in its ovary. In the case of Yersinia pestis infected fleas, they are not found to exhibit transovarial transmission.

Clinically plague infection in humans tends to be of the bubonic (affecting the lymphatic system) where it spreads and causes the characteristic “bubo” or the septicemic (blood bourne) type. Other rarer and more feared types of plague include pneumonic and meningeal plague.

If diagnosed early Yersinia pestis typically is response to antibiotic treatment with Gentamicin or Doxycycline. The main means of prevention though is avoidance of contact with animal resevoirs such as squirrels, chipmunks and other rodent found around campsites.

Perry and Fetherston conclude:

Consequently,
maintenance of plague in nature is absolutely dependent
upon cyclic transmission between fleas and mammals.